about define_diffusivity

shuqin

I am modeling the transport of multi-conponents along two different zones,and the following code for setting different diffusivities : DEFINE_DIFFUSIVITY(mass_diff, c, t, i) {

if (THREAD_ID(t)==ZONE_1)

D = 12345;

if (THREAD_ID(t)==ZONE_2)

D = 67890;

return D; } then what does the value D stands for,is it the diffusivity for species i in the mixture?if it is the case ,i suppose the Fick's law approximation is adopted by default,how should i set the binary diffusivity with UDF if i want to model in full multicomponent model?